The present invention relates to media compatible pressure sensing devices and methods for their fabrication. More specifically, this invention relates the design of a media compatible pressure-sensing device capable of high sensitivity to pressure with improved reliability.
Measuring pressure in ultra clean environments or environments containing harsh media has always been a challenge. There has been a continuous effort to produce affordable, reliable, media compatible pressure sensors.
Originally metal strain gauges were prevalent. These strain gauges comprised four resistors arranged in a Wheatstone bridge configuration such that two opposite resistors would increase in resistance with applied pressure and the other two would decrease with applied pressure. The resistance change was due to the dimensional changes in the metal resistors.
Other circuit configurations have also been used, but the four-resistor Wheatstone bridge configuration is still prevalent.
The more sensitive silicon micro electromechanical system (xe2x80x9cMEMSxe2x80x9d) based devices replaced many of the metal strain gauges. In these devices, resistors are formed within a silicon diaphragm by ion implantation or diffusion. These resistors exhibit a piezoresistive effect such that two opposite resistors increase in resistance and two decrease in such a way that each output changes in opposite ways. Metal (such as aluminum) is applied to the diaphragm for interconnects and pads for wire bonding. MEMS devices often require protection to make them media compatible.
Metal, glass, ceramic, plastic, or other chemically compatible diaphragms are used to protect silicon pressure sensors from harsh media. When using such diaphragms, a fluid (such as oil) is used to transfer the pressure from the chemically compatible diaphragm to the silicon diaphragm.
Some applications cannot tolerate the chance of an oil leak if there were to be some sort of diaphragm rupture. In this case, silicon strain gauges are used. Silicon strain gauges are often relatively long and thin. They are fragile, difficult to match and difficult to handle. Their use requires them to be attached directly to a metal, glass, ceramic, or plastic diaphragm.
A silicon chip can also be attached directly to a metal, glass, ceramic, or plastic diaphragm. In this case, all four resistors can be placed on one chip. These chips are less fragile, easier to position, and intrinsically matched. However, all four resistors must necessarily be very close together. Optimizing performance by judiciously locating resistors on the diaphragm cannot be done without adding additional Wheatstone bridge circuits that can make temperature compensations more difficult.
All of the above technologies are commercially available today.
A pressure transducer in accordance with the present invention comprises a novel pressure sensing structure. This structure includes a body of a first material within which a diaphragm is constructed. The diaphragm, in one embodiment contains a relatively thick boss centrally located. This diaphragm provides media isolation from the sensor. One or more pressure sensing elements is attached to the diaphragm with a second material. Each pressuresensing element comprises a triangular chip with one or more strain-sensing elements on it. In one embodiment, the triangular chip is a semiconductor material such as silicon. Typically, each strain-sensing element is a resistor that is electrically isolated by a dielectric layer in a silicon-on-insulator structure. Dielectric isolation enables performance at higher temperatures. Alternatively, junction isolated resistors can be used. The pressure sensing chips are located close to areas of maximum absolute stress. The triangular shape improves the reliability.
The resistors are typically piezoresistive. In lieu of resistive strain sensing elements, in other embodiments, capacitive strain sensing elements are used. In such an embodiment, stress changes the capacitance exhibited by the capacitive strain sensing elements.
In another embodiment, piezoelectric strain sensing elements are used. Stress changes the voltage across the piezoelectric strain sensing elements.